We collected various substrate and water samples during the late summer and autumn of 1991 from acid sensitive Faylor Lake in Union County, Pennsylvania. The substrate samples included sediment, bark shavings from submerged wood, and aquatic macrophyte leaves. These samples, taken from September through November, were fixed in a 0.5% osmium tetroxide solution and observed with a scanning electron microscope. The water samples, collected from January to November, were preserved with Lugol's iodine and observed under light microscopy. Our purpose was to examine the life cycles of the dinoflagellate speciesPeridinium inconspicuum.Peridinium wisconsinense. andPeridinium gatunense. The water samples showed that dinoflagellate populations peaked in late summer and then quickly disappeared from the water column. The substrate data showed that the differentPeridiniumspecies showed a preference for specific substrates prior to zygote formation. This suggests a selective function for the planozygote stage during the life cycle.